lanciani



Aug. 11, 1959 D. A. LANCIANI 2,899,651

WAVEGUIDE BEND Filed Jan. 5, 1955- 2 Sheets-Sheet 1 6d 50 IO g. 2 3

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INVENTOR. {L DANIEL A. LANCIANI BY )fi m WMWJDUW ATTORNEYS Aug; 11, 1959 D. A. LANCIANI V WAVEGUIDE BEND 2 Sheets-Sheet 2 Filed Jan. s, 1955 IIlI/l/l/ll).

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DANIEL A. LANCIANII BY ,wpfli w M I Fig. 9.

ATTORNEY S I i i 2,899,651 unlted Siatfis Patent Ce Be se se A95- 12. 9

WAYEGP PE 5 Daniel A. .Lanciani, West Medici-d, Mass, assignor to M crowa Assqc i s, inc liq g Ma s a 90mm:

i iqdfhi f iiil rs t hulk? t enfitisl swi i r m ci ina tq ri l n u wer fguidefrnake'thesharp' e I or w my ili fiefreciangum si i e an? t en" here s we t nsi on b l c it 'tiqi Q' i nwau q n ma e i pdss te t i 001 or less ior'wav proportionately s'rnaller radii at a und r tand n oi the T p e a 7 results are achieved Will be facilita't I I o t d sw nss Whi h I angle bend according to Fig.

The invention achieyes the .aboveqrrentioned objectives vby means of a pair of rectangularwaveguides inga cruciform cross-sectionin'wh ch there are generated W9 a e T1320 mode in z crqjs ed'relationship. The

generation'bf this modein acruciforrn cross-section's preferably achieved by means of a direct coaxial tr a tion fromthe ircular wavegu d swhi h h TEmm de is being propagated. 'Ihisfconstruction shown in perspectiyejin Fig '1 and sectional views in Figs. 2 and 3. As illustrated in thesel'y'iews, the circular waveguides 1, 0 and'2 .0 are in axial'fi i tratipn'with a section of rectangula'r waiteguide 3.0 containing a cruciform cross-section 50. The transition between the two types of sections preferably employs on each side a short tapered section 5.0 vfrom theflcircuni'ference of thetcircularguidle to therouter edgelof .thecruciforrnfollowed b3 13 Conical cutaway section Q0 in which the transition frtnn TEB 16m and back again is'jmadeqgradually in 'a structure compatiblewith'both modes, A Principal "ady antage of thisapparatus is the facility with which this transition can be made from one .modefl'to theother em oying if d ired the conical or, ,pyramida1"transition having 119 abrupt discontinuities to fest viinwa'ritedtmodes. 1 he transitionrnay, of course, besteppe n'aceordancewith copy'eritional design procedures? i V An' understaiiding ofthis apparatus depends upon an under ib t i de ei r a' a iidii Se tions The TE 'inodei s characterized by a cireular'felectric field around lthe axis lofthe circular waiteguideas shown in Fig. 4: This configuration is one inwhich none ofthe electric .fielcl lines terr ninatefat the Wa s f th guide and h ma netic {fie d si the W -$14K f a'ce is entirely longitudinal. As a resultionly transverse currents flow'in the guide walls and the a noiintbf termini ed b ii p eet i'" nducting w ll i markedly reduced. Unrortunately theT-Eoi wave has"; ljlatively short cut-tiff ,lengthje qual {to approximately .82 .tir nes the guide diameter and there are fouriwa yeso'f e ua 9 eat r u i fi W e h whi h m he P Pegated simult mq sly unless the" design is carefully coin- Il wi 9 i i eet h and ERE 1 simple iweve ui ietend qfi l -l iwi lli i mi 5 1 1 tapers in the circular guide, particularly at higher fie,- quences have been very difficult to operate successfully because of the tendency toward the production of higher order TE modes, where thelatter are possible propagating modes. s v

The TE mode in a rectangular Waveguide is illustrated in Fig. 5 and on superficial inspection would not appear to be in any way related to'the TE mode in a circular guide either in appearance or in the shape of the structure iifwhich will propagate. Howeverfit W-illlie apparent fi 'oi'n' a more detailed consideration or the circular waveguide in-F-ig. f} and the'rectangular waveguide 'ofFigl that if the electrical field Vectors .arc p'ppeny superimposed" the two modes can be rnade'tE) haves u'seful'relationshipl p -Tihe inyeiitio "creates such a relationship by th'e'use reetap mar waveguides pro agating of'ndtbfli'ljutm s i t what is' substantially the *TE H mode: thsetwowaire guides being formed to create a single cruciform opening. Such a cross-section is illustrated in Fig. 7. Propagation in such a cruciform takes place substantially independently in the two waveguides formed by the two opposed pairs of arms in the cruciform and each propagates a mode very similar to the pure TE mode in ordinary rectangular guides. To achieve the transition from the circular to the cruciform cross-section and back again it is preferable to use the shallow taper and transformer section referred to above. A direct transition might be possible but the formation of unwanted modes by a direct discontinuity makes an abrupt change in cross-section undesirable. To avoid this difficulty the transition 66 is employed which is formed by cutting away the solid material forming the arms of the cruciform to create a hollow conical cavity. For convenience the slope of the cone may be a continuation of the slope of the short taper section. The arms of the cruciform extend from the surface of the cone parallel to the axis of the cone and as the cone decreases in diameter a greater proportion of the microwave energy is propagated in the cruciform arms. Fig. 6 illustrates the simultaneous propagation in the circular and cruciform Waveguides midway along the transition. Although the transition is illustrated in the form of a smoothly tapered cone it would be of course equivalent to use a stepped conical or pyramidal cavity designed for the specific wavelength in accordance with conventional practice in stepped transformers.

While a transition section is used in the preferred embodirnent of this device, one of the features of this invention is the fact that the cruciform and the circular waveguide differ very little in maximum dimension and the one can be fed directly into the other. The discontinuities in the taper and the transition structure are minimized in this device and therefore the formation of spurious modes is avoided.

It is possible in the cruciform cross-section to use a mitered corner and re-taper back to the circular crosssection as illustrated by Figs. 8 and 9. However, this abrupt discontinuity tends to produce an excessive amount of mode impurity. Instead the preferred embodiment of the invention uses a curve in the cruciform cross-section, said curve having a radius large enough to minimize the differential phase shift between the E plane and the H plane portions of the bend. It can be shown that where there are E plane and H plane bends in rectangular waveguides of identical radius, the H plane bend will produce a greater phase shift from the equivalent E plane bend. In practice it can be shown that the guide wavelength of the H plane bend decreases with decreasing radius whereas the E plane bend wavelength is relatively unaffected. However, a shift in the H plane is not substantial until the radius becomes quite small. In fact, for X band standard waveguides it can be shown that the phase shift does not begin to rise substantially until the radius of the bend becomes less than one foot.

The difference in phase shift between an E plane and an H plane rectangular waveguide bend of identical radii can be derived from the formula and is equal to kgH AgE' The relative shift is thus a function of path length radius and guide wavelength. Assuming identical radii for the path length center lines in the cruciform section, it is apparent that bending radii on the order of one foot or less are possible for frequencies higher than 8 kilomegacycles, based on an acceptable level of mode impurities arising from unequal electrical path lengths. The predominant circular waveguide mode impurity arising from the unequal path lengths wouldbe the TE mode.

Since the cruciform should have a cut-off wavelength approximately the same as that of the main circular waveguide, it is a relatively straightforward matter to calculate the dimensions of the cruciform once the dimensions of the circular waveguide have been determined. A circular waveguide propagating the TE mode has a cut-off wavelength equal to .82D where D is the waveguide diameter. The cut-ofi wavelength for the oruciform will be very close to that of the TE mode in a single rectangular waveguide which is equal to the wide dimension of such a waveguide. However it has been found that the cutoff wavelength in the cruciform is on the order of .9 in the wide dimension. It will be seen that with identical dimensions these two cross-sections would be very closely matched and would not need any taper, although the conical transition would still be desirable.

In the taper and transition sections from circular to cruciform cross-section a taper and transition of at least four times the wavelength being propagated was found to be desirable in order to minimize unwanted reflections. The overall insertion loss in this bend is approximately A db over a greater than 6% band width and the loss for a bendin the 9.375 kmc. band is less than db over the same percentage band width. In addition the use of these bends results in a mode purity of approximately 20 db.

An alternative construction for a bend of maximum compactness is illustrated in Figs. 8 and 9. In this construction the tapered section 82 and the transition section composed of the cruciform superimposed on a conical cavity 89 are the same as those shown for the gradual bends. However the cruciform is constructed in two straight sections 86 and 88 intersecting at right angles. The corner is mitred at 45 to the axis of each of the two intersecting cruciform sections and a conducting plate 90 is placed in the plane of the mitre cut. While the operation of this form of the invention is essentially the same as the gradual bend, there is a greater phase shift between the reflected waves between the E and H planes and somewhat greater tendency to produce spurious modes.

While this invention has been described in respect to two specific embodiments it will be understood that the invention is not restricted to these embodiments but rather to the principles disclosed herein as defined by the following claims.

I claim:

1. Apparatus for changing the direction of the TE mode in a circular waveguide which comprises a transition structure from circular waveguide to a waveguide having a cruciform cross-section in the shape of two intersecting rectangles of essentially constant width, a bent section of cruciform rectangular waveguide having the said cross-section, and a transition section transforming back from such cruciform to circular cross-section.

2. Apparatus for changing the direction of microwave energy propagating in the TE mode which comprises two circular waveguides capable of propagating the TE mode, two transition structures from circular Waveguide to cruciform waveguide, joined by a curved cruciform waveguide whose cross-section has the shape of two intersecting rectangles of essentially constant width with the length of each arm of the cruciform waveguide of the same order of magnitude as the radius of the circular waveguide and having the axis of the cruciform waveguide in registration with the axes of the circular waveguides at either end.

3. Waveguide apparatus for propagating the TE mode around a bend which comprises two circular waveguides, a curved section of waveguide having a cruciform cavity in cross section in which two crossed modes resembling the TE mode of rectangular waveguide are propagated and a pair of axially symmetric transition sections at either end of said bend in axial alignment with the cruci,

form cross-section and with the circular waveguides whereby the TE mode transfers energy to the 'IE mode on entering the cruciform bend and transfers energy back to the TE mode on leaving the cruciform.

4. Waveguide apparatus for changing the direction of TE mode microwave energy propagating in circular waveguides with minimized attenuation which comprises two circular waveguides for propagating the TE mode, a bent cruciform waveguide whose cross-section has the shape of two rectangularly intersecting rectangles of essentially constant width, and two transformer sections between the circular and cruciform waveguides, said transformers being symmetrical in four quadrants about the axis of the circular waveguide and transferring energy from the TE mode in the circular waveguide to the TE mode in the cruciform cross-section.

5. Waveguide apparatus comprising a circular waveguide, a cruciform waveguide whose cross-section has the shape of two rectangularly intersecting rectangles of essentially constant width in axial alignment with the circu lar waveguide, and a transition structure separating the two waveguides and having a pyramidal cavity tapering between parallel walls from the center of the cruciform waveguide to the outer surface of the circular waveguide said cavity having slots along its surface which are in axial registration with the arms of the cruciform waveguide.

6. Waveguide apparatus for propagating the TE mode around a bend which comprises a circular waveguide, a tapering transition section having a length equal to at least three times the propagated wavelength, a pair of superimposed rectangular waveguides forming a cruciform having its axis in registration with the axis of the circular waveguide and having its widest dimension substantially equal to the widest dimension of the circular waveguide in order to propagate the TE mode, a bend in the cruciform cross-section having a radius substantially larger than four times the propagated wavelength and a second identical tapered transition from the cruciform to circular cross section whereby the TE mode is reestablished in the circular waveguide.

7. Apparatus for changing the direction of energy propagating in the TE mode which comprises two circular waveguides, two transition structures from circular waveguide to cruciform waveguide, two cruciform waveguides whose cross-sections have the shape of two intersecting rectangles of essentially constant width in axial alignment with the circular waveguides and intersecting at right angles, and a conductive end plate at to both axes through their point of intersection in a plane at right angles to the plane formed by the two axes.

8. Waveguide apparatus for propagating the TE mode around a bend which comprises: a circular waveguide; a first radially tapering transition section having substantially parallel longitudinal walls and having a length equal to at least three times the propagated wavelength; a pair of superimposed rectangular waveguides forming a cruciform having its axis in registration with the axis of the circular waveguide and having its widest dimension substantially equal to the widest dimension of the circular waveguide in order to propagate the TE mode, a circular bend in the cruciform cross section having a radius substantially larger than four times the propagated wavelength; and a second transition section, substantially identical to said first transition section, from the cruciform to circular cross section; whereby the TE mode is reestablished in the circular waveguide, the transition sections including axially tapering conical wall sections extending between the respective circular waveguides and the respective sides of the cruciform.

References Cited in the file of this patent UNITED STATES PATENTS 2,439,285 Clapp Apr. 6, 1948 2,441,574 Jaynes May 18, 1948 2,649,578 Albersheim Aug. 18, 1953 2,706,278 Walker Apr. 12, 1955 

